1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly, to a reflective cholesteric liquid crystal (CLC) display device and a manufacturing method for the same.
2. Discussion of the Related Art
Flat panel display devices, which have properties of thin, low weight and low power consumption, have been required as the information age rapidly evolves. The flat panel display device may be classified into two types depending on whether it emits light or not. One is a light-emitting type display device that emits light to display images and the other is a light-receiving display device that uses an external light source to display images. Plasma display panels (PDPs), filed emission display (FED) devices and electro luminescence (EL) display devices are examples of the light-emitting type display devices and liquid crystal displays are an example of the light-receiving type display device. The liquid crystal display device is widely used for notebook computers and desktop monitors, etc. because of its superior resolution, color image display and quality of displayed images.
Generally, the liquid crystal display device has upper and lower substrates, which are spaced apart and facing each other. Each of the substrate includes an electrode and the electrodes of each substrate are facing each other. Liquid crystal is interposed between the upper substrate and the lower substrate. Voltage is applied to the liquid crystal through the electrodes of each substrate, and thus an alignment of the liquid crystal molecules is changed according the applied voltage to display images. Because the liquid crystal display device cannot emit light as described before, it needs an additional light source to display images. Accordingly, the liquid crystal display device has a back light behind a liquid crystal panel for a light source. An amount of light incident from the back light is controlled according the alignment of the liquid crystal molecules to display images. The electrodes of each substrate are formed of transparent conductive material and the substrates must be transparent. The liquid crystal display devices like this are called transmissive liquid crystal display devices. Because the transmissive liquid crystal display device uses an artificial light source such as the back light, it can display a bright image in dark surroundings. However, the transmissive liquid crystal display device has high power consumption.
The reflective liquid crystal display device has been suggested to overcome the power consumption problem of the transmissive liquid crystal display device. Because the reflective liquid crystal display device controls a transmittance according the alignment of liquid crystal molecules by irradiating light using an external light source such as ambient light or artificial light, it has a low power consumption compared with the transmissive liquid crystal display device. An electrode of the lower substrate is formed of conductive material, which has a high reflectance and an electrode of the upper substrate is formed of transparent conductive material to transmit the incident light.
The conventional reflective liquid crystal display device will be described hereinafter more in detail with reference to FIG. 1. FIG. 1 is a cross-sectional view of a conventional reflective liquid crystal display device. In FIG. 1, a plurality of switching elements (not shown) are formed in an array matrix on a first substrate 1 and a plurality of reflective electrode 3, which are connected to each of the switching elements, is formed on the first substrate 1. The reflective electrode 3, which is formed of conductive material such as metal, serves to reflect the incident light and serves as a pixel electrode. A color filter 4 that includes sub-color-filters red (R), green (G), and blue (B) in a repeated order is formed beneath a second substrate 2 and corresponds to the reflective electrode 3. A common electrode 5 is formed of transparent conductive material beneath the color filter 4. Liquid crystal is interposed between the reflective electrode 3 and the common electrode 5. An alignment of liquid crystal molecules is changed if a voltage is applied to the reflective electrode 3 and the common electrode 5. An alignment film (not shown) is respectively formed on the reflective electrode 3 and beneath the common electrode 5 to align the liquid crystal molecules into a uniform direction.
A retardation layer 7 is formed on the second substrate 2. The retardation layer 7 here in the figure has a phase difference of λ/4 and thus is called a quarter wave plate. The quarter wave plate 7 changes a linear polarization of light into a circular polarization of light and the circular polarization into the linear polarization. A polarizer 8, which changes ambient light into linearly polarized light by transmitting only the light that is parallel to a light transmission axis, is formed on the quarter wave plate 7. If the ambient light is irradiated to the reflective liquid crystal display device when the voltage is not applied, the incident light is changed into linearly polarized light as it passes through the polarizer 8, and the linearly polarized light is changed into circularly polarized light as it passes through the quarter wave plate 7. The circularly polarized light then passes through the second substrate 2, the color filter 4 and the common electrode 5 in sequence and there is no phase change during this process. The circularly polarized light then passes through the liquid crystal layer 6 and is changed into linearly polarized light as it passes through the liquid crystal layer 6 if the liquid crystal layer 6 is formed to have a phase difference of λ/4. The linearly polarized light is reflected at the reflective electrode 3 and then is changed into circularly polarized light as it passes again through the liquid crystal layer 6. The circularly polarized light is changed into the linearly polarized light as it passes again through the quarter wave plate 7, and then the linearly polarized light passes through the polarizer 8. At this time, if a polarized direction of the linearly polarized light is parallel to the light transmission axis of the polarizer 8, all of the linearly polarized light transmits through the polarizer 8 and if the polarized direction of the linearly polarized light is perpendicular to the light transmission axis of the polarizer 8, the linearly polarized light cannot transmit through the polarizer 8.
On the other hand, cholesteric liquid crystal (CLC) display devices, which use CLC color filter to display color images, has been widely researched and developed in the field. The reflective cholesteric liquid crystal display device, which has CLC color filter, has a superior color display ability and contrast ratio compared with a typical reflective liquid crystal display device that has an absorption type color filter. The cholesteric liquid crystal color filter uses a selective reflection property of the cholesteric liquid crystal. The cholesteric liquid crystal functions as a reflective mirror when each layer of helical structure has a perfect alignment. That is, when all helical axes of the cholesteric liquid crystal are aligned perpendicular to the substrate, the cholesteric liquid crystal functions as the reflective mirror in which the incident light is reflected at a surface of the mirror and an incidence angle and a reflection angle are same. The cholesteric liquid crystal does not reflect all incident light, but selectively reflects the incident light of a particular wavelength according to a helical pitch of the cholesteric liquid crystal. Accordingly, the reflected light may display red, green or blue color by controlling the helical pitch according to each region of the CLC color filter. The cholesteric liquid crystal color filter also determines a polarization state of the reflected light. The rotational direction of the cholesteric liquid crystal helix is an important factor to make a polarization phenomenon. For example, the left-handed cholesteric liquid crystal reflects a left circular polarization that has a wavelength corresponding to the pitch of the left-handed cholesteric liquid crystal. That is, a direction of a circular polarization of the reflected light depends on whether the helix structure of the cholesteric liquid crystal is right-handed or left-handed. This is a great difference from a typical dichroic mirror that simply reflects light of particular wavelength and transmits the rest of the light.
FIG. 2 is a cross-sectional view of a reflective cholesteric liquid crystal display device that has a CLC color filter according to the related art. Because the cholesteric liquid crystal color filter serves as a reflector as well as a color filter, an additional reflector is not needed. In FIG. 2, an absorption layer 12 is formed on the lower substrate 11 and a first alignment layer 13 is formed on the absorption layer 12. A cholesteric liquid crystal color filter 14 is formed on the first alignment layer 13. The cholesteric liquid crystal color filter 14 displays red, green or blue color by reflecting light that has a wavelength corresponding to the red, green or blue color, respectively. A transparent first electrode 15 is formed on the cholesteric liquid crystal color filter 14, and a second alignment layer 16 is formed on the first electrode 15. A transparent second electrode 22 is formed beneath an upper substrate 21, and a third alignment layer 23 is subsequently formed beneath the second electrode 22. A retardation layer 30 that has a phase difference of λ/4 is formed on the upper substrate 21, and a polarizer 40 is formed on the retardation layer 30. A liquid crystal layer 50 is positioned between the second alignment layer 16 and the third alignment layer 23. The alignment of liquid crystal molecules is changed according to an applied voltage between the first electrode 15 and the second electrode 22.
A driving mechanism of the reflective cholesteric liquid crystal display device, which uses a cholesteric liquid crystal color filter, is as follows. A phase difference in the liquid crystal occurs when the voltage is applied to the liquid crystal.
In case of normally black mode, when the voltage is not applied to the liquid crystal, incident light is linearly polarized as it passes through the polarizer 40 and subsequently circularly polarized as it passes through the retardation layer 30. The circularly polarized light passes through the liquid crystal layer 50 without a phase retardation and then transmits through the cholesteric liquid crystal color filter without reflection, and finally is absorbed in the absorption layer 12. Accordingly, there is no reflected light. Whereas, when the voltage is applied to the liquid crystal, incident light is linearly polarized as it passes through the polarizer 40 and subsequently circularly polarized as it passes through the retardation layer 30. The polarization property of the circularly polarized light is changed because of phase retardation as it passes through the liquid crystal layer 50. Only light of particular wavelength in the light that passes through the liquid crystal layer 50 is reflected at the cholesteric liquid crystal color filter 14, and the rest of the light transmits through the cholesteric liquid crystal color filter 14 and then is absorbed to the absorption layer 12. The polarization property of the reflected light is changed as it passes again through the liquid crystal layer 50, and the reflected light is linearly polarized as it passes through the retardation layer 30. The linearly polarized light finally passes through the polarizer 40.
In case of normally white mode, when the voltage is not applied to the liquid crystal, incident light is circularly polarized as it passes through the polarizer 40 and the retardation layer 30. The circularly polarized light passes through the liquid crystal layer 50 without phase retardation. Only light of a particular wavelength of the light that passes through the liquid crystal layer 50 is reflected at the cholesteric liquid crystal color filter 14, and the rest of the light transmits through the cholesteric liquid crystal color filter 14 and then is absorbed in the absorption layer 12. The reflected light passes again through the liquid crystal layer 50 without phase retardation and is linearly polarized as it passes through the retardation layer 30. The linearly polarized light finally passes through the polarizer 40. Whereas, when the voltage is applied to the liquid crystal, incident light is circularly polarized as it passes through the polarizer 40 and the retardation layer 30, and the polarization property of the circularly polarized light is changed because of the phase retardation as it passes through the liquid crystal layer 50. All of the light, that has passed through the liquid crystal layer 50 passes through the cholesteric liquid crystal (CLC) color filter 14 without reflection and then is absorbed to the absorption layer 12. Accordingly, there is no reflected light.
Because the reflective liquid crystal display device uses an external light source, an incidence angle of the light varies according to a position of the external light source. As described before, because the cholesteric liquid crystal color filter does a specular reflection, the reflection angle of the light depends on the incidence angle of the light. Accordingly, whereas a luminance in a certain viewing angle is very high, the luminance in the rest of viewing angle is lowered. In addition, because a size of the pitch of the cholesteric liquid crystal (CLC) helix, which the incident light experiences, is varied according to the incidence angle of the incident light, the wavelength of the reflected light is changed. Accordingly, a color of the reflected light varies depending on the incidence angle of the incident light and a change of color of the reflected light becomes greater as the incidence angle becomes bigger. These problems can be overcome by scattering the reflected light using a diffusion film over the liquid crystal panel, which helps to uniform the luminance in a main viewing angle range. Though an introduction of the diffusion film may overcome the luminance problem, there still exists a color change problem according to the incidence angle.